#coding:utf-8

class Subject(object):
    def __init__(self):
        self._observers = []
    
    def attach(self, observer):
        if not observer in self._observers:
            self._observers.append(observer)
    
    def detach(self, observer):
        try:
            self._observers.remove(observer)
        except ValueError:
            pass
    
    def notify(self, modifier=None):
        for observer in self._observers:
            if modifier != observer:
                observer.update(self)


# 用法示例
class Data(Subject):
    def __init__(self, name=''):
        Subject.__init__(self)
        self.name = name
        self._data = 0
    
    @property
    def data(self):
        return self._data
    
    @data.setter
    def data(self, value):
        self._data = value
        self.notify()


class HexViewer:
    def update(self, subject):
        print('HexViewer: Subject %s has data 0x%x' % (subject.name, subject.data))

        
class DecimalViewer:
    def update(self, subject):
        print('DecimalViewer: Subject %s has data %d' % (subject.name, subject.data))


# Example usage...
def main():
    data1 = Data('Data 1')
    data2 = Data('Data 2')
    view1 = DecimalViewer()
    view2 = DecimalViewer()
    data1.attach(view1)
    data1.attach(view2)
    data2.attach(view2)
    data2.attach(view1)
    
    print('Setting Data 1 = 10')
    data1.data = 10
    print('Setting Data 2 = 15')
    data2.data = 15
    print('Setting Data 1 = 3')
    data1.data = 3
    print('Setting Data 2 = 5')
    data2.data = 5
    print('Detach HexViewer from data1 and data2')
    data1.detach(view2)
    data2.detach(view2)
    print('Setting Data 1 = 10')
    data1.data = 10
    print('Setting Data 2 = 15')
    data2.data = 15


if __name__ == '__main__':
    main()
